DFU mode: improve programming robustness.

The ctrl_transfer in dfu.get_status() might return usb.core.USBError
with errno == EPIPE, seen when called in quick succession. This is the
case when called from dfu.block_on_state(). The next call usually works.

This change tries to catch this specific error and just re-tries up to
two times in case it happens.

Without this change, the error (and programming abort) might get
undetected, as 'click' catches IOerror/EPIPE silently. This has
significant potential to brick a device if you don't check the output
and return code of 'solo' closely enough.

solo/dfu.py

@@ -7,6 +7,7 @@
 # http://opensource.org/licenses/MIT>, at your option. This file may not be
 # copied, modified, or distributed except according to those terms.
 
+import errno
 import struct
 import time
 
@@ -137,10 +138,24 @@ def close(self,):
         pass
 
     def get_status(self,):
-        # bmReqType, bmReq, wValue, wIndex, data/size
-        s = self.dev.ctrl_transfer(
-            DFU.type.RECEIVE, DFU.bmReq.GETSTATUS, 0, self.intNum, 6
-        )
+        tries = 3
+        while True:
+            try:
+                # bmReqType, bmReq, wValue, wIndex, data/size
+                s = self.dev.ctrl_transfer(
+                    DFU.type.RECEIVE, DFU.bmReq.GETSTATUS, 0, self.intNum, 6
+                )
+                break
+            except usb.core.USBError as e:
+                if e.errno == errno.EPIPE:
+                    if tries > 0:
+                        tries -= 1
+                        time.sleep(0.01)
+                    else:
+                        # do not pass on EPIPE which might be swallowed by 'click'
+                        raise RuntimeError("Failed to get status from DFU.")
+                else:
+                    raise
         return DFU.status(s)
 
     def state(self,):